[jira] [Commented] (HDFS-3862) QJM: don't require a fencer to be configured if shared storage has built-in single-writer semantics

["Todd Lipcon (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-3862?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13443518#comment-13443518 ] 

Todd Lipcon commented on HDFS-3862:
-----------------------------------

I think this might be the case for BookKeeper as well. Any of the folks working on BKJM want to take this on? I anticipate we would add a simple API to JournalManager like: {{boolean isNativelySingleWriter();}} or {{boolean needsExternalFencing();}}. Then the failover code could check the shared storage dir to see if this is the case, and if so, not error out if the user doesn't specify a fence method.
                
> QJM: don't require a fencer to be configured if shared storage has built-in single-writer semantics
> ---------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-3862
>                 URL: https://issues.apache.org/jira/browse/HDFS-3862
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: ha
>    Affects Versions: QuorumJournalManager (HDFS-3077)
>            Reporter: Todd Lipcon
>
> Currently, NN HA requires that the administrator configure a fencing method to ensure that only a single NameNode may write to the shared storage at a time. Some shared edits storage implementations (like QJM) inherently enforce single-writer semantics at the storage level, and thus the user should not be forced to specify one.
> We should extend the JournalManager interface so that the HA code can operate without a configured fencer if the JM has such built-in fencing.

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